This invention relates generally to optical communication systems and components. More particularly, the invention relates to optical systems comprising micro- and nano-scale optical structures and components built from and upon monolithic substrates.
In the field of telecommunication, it is recognized that optical communication components and systems, which use light waves and beams to carry information, offer many considerable advantages over conventional copper wire-based communication systems that carry information in the form of electrical signals. One advantage is the significantly greater amount of information that can be carried by a physical connection employing a single fiber optic strand as compared to a copper wire circuit.
To optimize the amount of information that can be transmitted along a single optical fiber, the technique known as Dense Wavelength Division Multiplexing (DWDM) is now being implemented. The use of DWDM is accelerating at a rapid pace, due to the development of essential network components such as optical fiber, infrared laser transmitters, fiber amplifiers, and the like. However, the rate of growth of DWDM networks is currently limited in part by the availability of low-cost mass produced components that provide acceptable reliability and resistance to environmental effects, such as, vibration, mechanical stresses, and temperature fluctuations.
In these optical telecommunications systems, information is transmitted in the form of infrared light signals that originate at laser sources. Each laser source is tuned to emit an infrared light beam comprising a narrow band of wavelengths (or, equivalently, frequencies) centered at a primary frequency. As used herein, the word xe2x80x9ccolorxe2x80x9d describes a characteristic band of wavelengths centered around a specific primary wavelength as emitted by a telecommunications laser. Information is encoded in each infrared beam by temporally modulating the laser power. Each primary frequency corresponds to a value specified by the standard International Telecommunication Union (ITU) grid. The ITU standard specifies transmission frequencies that are spaced at 100 GHz intervals, although a reduction to 50 or 25 GHz is anticipated as the technology evolves.
In DWDM, many distinct colors of infrared light may be transmitted simultaneously along a single optical fiber, each color carrying information that is distinct from information carried by other colors. Devices called multiplexers physically superimpose the light beams from several colors thereby creating multi-color light beams wherein each color carries its encoded information. The combined information is transmitted some distance. At a terminus of the transmission path, demultiplexers physically separate or disperse the multiple colors received from a single optical fiber onto multiple output fibers, each output fiber carrying a single color, thereby permitting the information carried by each color to be directed to its intended destination.
The ideal demultiplexer will direct all of the incoming light of each color onto its corresponding output optical fiber. However, in actual demultiplexer devices the color separation is generally imperfectxe2x80x94not all of the light of each color entering the demultiplexer is transmitted into each respective output beam, and a portion of the light from each color is transmitted into the paths of neighboring beams. This leakage causes undesirable performance effects such as crosstalk and insertion loss that must be limited in magnitude for the device to be practical.
At present, demultiplexing is often accomplished utilizing devices based upon either diffraction gratings or precision interference filters. Neither type of device is amenable to cost-effective mass production while maintaining acceptable performance.
For filter-based demultiplexers, such as those described in T. E. Stern, K. Bola, Multiwavelength Optical Networks, A Layered Approach, Addison Wesley, 1999, each filter must be manufactured separately from the others using multilayer vapor deposition techniques. The filters are then installed manually or robotically in an optical substrate and aligned to project light onto individual output optical fibers. Achieving and maintaining optical alignment in spite of thermal and mechanical stresses confounds attempts to reliably mass produce these devices, adding to the production cost and diminishing long-term reliability and resistance to environmental influences. Furthermore, the filters are frequently operated in a serial configuration, such that one color is transmitted through one filter while all other colors are reflected to the next, and so on. In this configuration, the insertion loss accumulates so that the last transmitted color has significantly higher loss than earlier colors.
Grating-based devices offer the advantage of being parallel rather than serial demultiplexers, and therefore have improved insertion loss uniformity. However, to achieve acceptable crosstalk, the optical components within grating-based devices must be several centimeters in size. Therefore, like filter-based devices, grating-based demultiplexers are difficult to align and maintain aligned. Low-cost mass production of reliable devices remains elusively difficult.
Recently, demultiplexers based on arrayed waveguide gratings (AWGs) have been introduced commercially. These small, thin monolithic devices, generally fabricated from silicon-based or InP-based wafer substrates, offer promise as low-cost components that can be mass produced. Nevertheless, despite more than a decade of intense development of AWG technology, and the emergence of several companies offering AWG products, the performance specifications achieved by production AWGs remain several orders of magnitude worse than theoretical predictions.
Monolithic demultiplexer devices based on waveguide gratings etched into wafers of semiconductor materials such as InP have been described in recent patent and technical literature. These devices also offer potential as low-cost mass-producible components but, like AWGs, have not yet achieved acceptable performance specifications. In particular, etched waveguide gratings demonstrated to date suffer from excessive crosstalk due to the small size and imperfections in fabrication of the grating structure.
The invention, in one embodiment, provides a miniature monolithic optical wavelength demultiplexer comprising an assembly of optical components built on a monolithic platform.
In one aspect, the invention relates to a miniature monolithic optical demultiplexer that includes a wavelength dispersing optical element that receives a multi-color light beam and separates the light spatially, providing a plurality of substantially single color optical beams, each substantially single color optical beam containing primarily a single color different from that of the other substantially single color optical beams. In some embodiments, a small portion of the light from other colors can remain in a substantially single-color optical beam after passing through the wavelength dispersing optical element. The demultiplexer also includes a wavelength filter array having at least one wavelength filter element. Each wavelength filter element receives one of the plurality of substantially single color optical beams and removes therefrom the portions of light from colors other than the primary color of that beam, thereby providing a purified single color output beam substantially free of light from other colors.
In some embodiments, one or more of the substrate, the wavelength dispersive optical element, and the wavelength filter array comprises at least one material selected from the group consisting of silicon, silicon monoxide, silicon dioxide, silicon-germanium alloys, silicon carbide, silicon nitride, and indium phosphide. In other embodiments, different materials can be employed. In one embodiment, the demultiplexer is preferably made from materials that are amenable to semiconductor fabrication processes.
In one embodiment, at least one of the wavelength dispersive optical element and the wavelength filter array elements comprises an optical waveguide. In one embodiment, at least one of the wavelength dispersive optical element and the wavelength filter array elements comprises a miniature free-space optical element. With regard to the invention, the term xe2x80x9cfree-spacexe2x80x9d refers to optical systems or components in which light travels in a fluid, such as air, or in a vacuum, rather than through a solid waveguide material.
In some embodiments, the demultiplexer can further comprise an input optical structure that receives the input beam. The input optical structure can comprise at least one material selected from the group consisting of silicon, silicon monoxide, silicon dioxide, silicon-germanium alloys, silicon carbide, silicon nitride, and indium phosphide. The input optical structure can comprise an optical waveguide, or a miniature free-space optical element. In some embodiments, the miniature monolithic optical demulitplexer further comprises an optical waveguide that communicates the input beam from an external source.
In some embodiments, the demulitplexer further comprises an array of output optical structures having at least one output element, said at least one output element transmitting an output beam. The array of output optical structures can comprise optical waveguides, or miniature free-space optical elements. The array of output optical structures can comprise at least one material selected from the group consisting of silicon, silicon monoxide, silicon dioxide, silicon-germanium alloys, silicon carbide, silicon nitride, and indium phosphide.
The demulitplexer can further comprise an optical waveguide that communicates the output beam from said output element to a photodiode.
In one embodiment, each color comprises a narrow band of wavelengths centered on a primary wavelength, which can be designated by the International Telecommunications Union as one of a set of discrete wavelengths to be utilized for optical telecommunications.
In some embodiments, the wavelength dispersive optical element is a selected one of a prism, a grating, or a grism. In some embodiments, the wavelength filter array is a selected one of an array of interference filters, an array of waveguide Bragg gratings, an array of Fabry-Perot interferometers, an array of resonantly-coupled waveguide structures, or an array of waveguide ring resonators.
In another aspect, the invention relates to a method of demultiplexing an optical beam having a plurality of spatially overlapping colors. The method comprises providing an assembly of miniature optical elements, the assembly comprising a monolithic substrate, a wavelength dispersive optical element fabricated on said monolithic substrate, and a wavelength filter array fabricated on said monolithic substrate. In addition, the method comprises receiving an input beam having a plurality of spatially overlapping distinct colors, dispersing the input beam, by use of the wavelength dispersive optical element, into a plurality of spatially-separated substantially single color optical beams, each beam containing primarily a single color that is different than the color of the other substantially single color optical beams, and filtering said at least one of the substantially single-color optical beams and removing therefrom colors other than the designated primary color of that beam, thereby providing at least one purified single-color output beam, said output beam being substantially free of other colors.
In some embodiments, the input beam is received from an optical waveguide external to said assembly. In some embodiments, the output beam is transmitted to an optical waveguide external to said assembly.
In yet another aspect, the invention features a method of fabricating a miniature monolithic optical demultiplexer. The method comprises utilizing semiconductor fabrication methods to create a wavelength dispersive element and an array of wavelength filtering elements upon a monolithic substrate, the combination of the wavelength dispersive element and the array of wavelength filtering elements providing a miniature monolithic optical demultiplexer. In some embodiments, the wavelength dispersive optical element and the wavelength filter array are formed by semiconductor fabrication processes. In some embodiments, the method further comprises fabricating an input optical structure by semiconductor fabrication processes. In some embodiments, the method further comprises fabricating an output optical structure by semiconductor fabrication processes.
The foregoing and other objects, aspects, features, and advantages of the invention will become more apparent from the following description and from the claims.